It is well known that a telephone line is subject to various types of faults from which sensitive components (e.g. integrated circuits) in interface circuits coupled to the telephone line must be protected. Examples of such faults are surges due to lightning strikes, power line crosses and/or a.c. induction from power lines, other externally applied voltages on the telephone line, and ground faults in which a wire of the telephone line is undesirably grounded. Such faults may be of various durations, for example they may be transient as in the case of lightning surges, they may be of significant but not prolonged duration, or intermittent, as in the case of power line crosses, or they may be of prolonged duration, as in the cases of a.c. induction and ground faults.
In order for a protection arrangement to be effective, it must respond to arbitrary fault conditions before there is any risk of damage being caused to the components of the line interface circuit, without responding inappropriately to conditions which may occur in normal operation of a telephone line interface circuit.
It is well known to protect against lightning surges by providing surge arrestors. As lightning surges have a short duration and occur relatively frequently, it is desirable that other protection facilities either not respond to such surges, or quickly respond and then quickly and automatically reset themselves for normal operation. Automatic, but not necessarily very rapid, resetting of protection facilities on removal of relatively long-duration fault conditions is also required.
These requirements present considerable obstacles to overcoming the problem of providing an effective protection arrangement.
In Rosch et al. U.S. Pat. No. 4,947,427 issued Aug. 7, 1990 and entitled "Protection Arrangement For A Telephone Subscriber Line Interface Circuit" them is described a protection arrangement in which protection relay contacts in series with the line wires are opened in response to an excessive current flowing on the line and through feed resistors connected between the relay contacts and other pans of the line interface circuit. When the relay contacts are open so that current flow is interrupted, the voltage on the line continues to be monitored via connections to the line wires on the line side of the protection relay contacts.
While such an arrangement has considerable advantages, the voltage monitoring when the protection relay contacts are open due to a ground fault is not able to detect when the ground fault is cleared, so that automatic resetting of the protection arrangement following a ground fault is not easily achieved. The relatively common occurrence of ground faults makes this a significant problem.
An object of this invention is to provide an improved automatic protection and recovery method for a telephone line interface circuit.